In the known ECMA-279 standard for DVD-R (“ECMA—Standardizing Information and Communication Systems: 80 mm (1.23 Gbytes per side) and 120 mm (3.95 Gbytes per side) DVD-Recordable Disk (DVD-R)”) and the ECMA 338 standard for DVD-RW (“ECMA—Standardizing Information and Communication Systems: 80 mm (1.46 Gbytes per side) and 120 mm (4.70 Gbytes per side) DVD Re-recordable Disk (DVD-RW)”) the address information to identify a position on an unrecorded disc is stored in the so-called land pre-pits (LPP). These pre-pits are situated in the lands, which are the areas that separate the recording grooves, i.e. the tracks, from each other. Both the grooves as the recordable area and the lands holding the land pre-pits are formed during the mastering process of the disc, i.e. they are available on an unrecorded/blank disc.
The grooves are circles or spirals with a small radial sine wave shaped deviation called track wobble as shown in FIG. 1. A respective track wobble signal can be detected using a track error signal such as the push-pull signal. Additionally, some filtering or AC-coupling is applied. As well, the land pre-pits are detected by using the push-pull signal, as this signal also shows the reflectivity change on the outer edges of the groove, i.e. the neighbouring lands.
The land pre-pits, which belong to the groove that is actually read or recorded, are always situated on one side of the groove. According to ECMA-279, they are embossed on a predetermined position versus the track wobble modulation of the track. The groove wobble is regarded as a sine wave and the phase relation between the groove wobble and the land pre-pit signals (PWP) is defined to be −90°±10°. Thus, the PWP value is measured as the phase between the largest point of the LPP-signal and the average zero crossing point of the wobble. The LPP search can accordingly be triggered by detecting the PWP value.
Furthermore the track wobble signal can be used for controlling the rotational speed of the optical recording medium during recording or playback. To create a reliable recording medium speed signal, a phase locked loop (PLL), which is locked to the track wobble, is typically used. The clock output of this PLL can be used to close a recording medium rotational speed loop. If this PLL is locked to the track wobble, also timing signals to control the land pre-pit detection block can be derived.
A simple detection system (FIG. 2) is proposed in ECMA-279 (Annex Q). As already mentioned the land pre-pit signal as well as the track wobble frequency is derived from the Push-Pull signal 1. The signal algebra to generate the Push-Pull signal is (A+B)−(C+D), wherein A, B, C and D are the four signals of a four-quadrant detector (not shown here) for detecting the position of the reflected laser beam. The push-pull signal is fed to a limiter 2, which cuts spikes which are above the typical push-pull amplitude, and is then processed by a band pass filter 3, which cuts off frequency components higher or lower than the nominal track wobble frequency. Then a voltage V1 is added. The result of this signal processing is compared with the original signal by a first comparator 4. A PLL (not shown) locks on the track wobble frequency. The PLL creates a locked sine wave signal having the same frequency and phase as the track wobble frequency. The PLL's output signal is compared with a second reference voltage V2 by a second comparator for creating a detection window. The outputs of the two comparators 4, 5 are combined via a noise gate 6, which typically performs a logical AND-operation.
A corresponding signal diagram is shown in FIG. 3. The curves A in FIG. 3 are the signals applied to the first comparator 4. The push-pull signal PPS has the typical sine wave form superimposed by some noise. At point P1 the PPS signal shows a distortion. At point P2 the PPS signal is deformed due to a land pre-pit. The second curve BPF+V1 in FIG. 3A represents the PPS signal filtered by the band pass filter 3 and shifted downwards by a constant voltage V1. The comparator 4 detects that the signal PPS is below the signal BPF+V1 at the points P1 and P2. The resulting comparator signal is given in FIG. 3B. In parallel the second comparator 5 compares a PLL signal having the same frequency as the PPS Signal with a constant voltage signal V2 as shown in FIG. 3C. As a result a second comparator 5 outputs a gate signal as depicted in FIG. 3D. An AND-operation of the signals of FIG. 3B and 3D leads to the signal shown in FIG. 3E. Thus, the comparator signal produced by the distortion at point P1 is eliminated by this AND-operation and a usable land pre-pit signal is obtained.
In the ECMA-279 system, the signal created by the land pre-pit pulse does not have a constant amplitude. Furthermore, it can be influenced by HF components due to imperfect common mode cancellation through the subtraction (A+B)−(C+D).
Also, influences on both track wobble amplitude and land pre-pit amplitude will occur caused by the neighbouring tracks. In case of the track wobble amplitude the effect is called wobble beat.
The land pre-pit detector proposed in ECMA-279 is based on the comparison of the limited and band passed signal compared with the original signal. If the amplitude of the track wobble signal rises above or falls significantly below the limiter values, the first comparator 4 might fail to detect the existing land pre-pits. Also, if the playback or recording speed is changed, as for example in a CAV mode, the BPF 3 needs to be tuned accordingly. A further drawback of the standard detector circuit is that the PLL (not shown) must create a synthesized sine wave of constant amplitude to enable the creation of a reliable noise gate signal.
In this connection the document EP 1 184 850 A2 discloses a recording and reproducing apparatus capable of detecting a land pre-pit on an optical disc securely with a high precision. One of the signals output from a split detector is multiplied with a predetermined coefficient, which is changed depending on an error rate upon decoding of land pre-pits, a lens shift amount of an objective lens, and the levels of the land pre-pits signal and a wobble signal relative to a data signal.